mmc: sdhci-spear: Initialize sdhci clk to 50 MHz

SPEAr sdhci driver expects the clock to be set to 50 MHz for proper
functioning.  This patch sets clk to 50 MHz in probe.

Signed-off-by: Vipul Kumar Samar <vipulkumar.samar@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>